This invention relates to a method of treating the spent caustic stream produced by a sour water stripper wherein the caustic stream contains quantities of sulfides. The method has utility in treating the sour water produced by a stripper, such as obtained from an olefin processing plant, to reduce the concentration of sulfides to the region of tenths of parts of sulfide per million parts of sour water.
Sour water is a common byproduct of industrial processes for obtaining raw materials used in the synthesis of various polymers. In the manufacture of polyethylene, for example, a chemical feed stock is pyrolyzed under conditions suitable for obtaining ethylene gas. The ethylene gas is subsequently polymerized to polyethylene. Typically, the chemical feed stock used in the process is natural gas or naphtha which contains a quantity of H.sub.2 S. As H.sub.2 S and other sulfides are not desired in the ethylene product and they are not destroyed in the pyrolysis process they must be stripped from the ethylene product stream. A wet air scrubber is used to remove the hydrogen sulfide by spraying an aqueous caustic solution into the ethylene product stream. The H.sub.2 S is absorbed by the caustic solution and ultimately leaves the stripper as the caustic sour water stream.
The sour water obtained from such stripping operations has typically been disposed of by deep well injection, that is, by injection into geological features deep underground. As concerns mount over the dollar cost and environmental impact of this disposal method it is becoming a less acceptable way of handling sour water.
Biological treatment of sour water has emerged as a possible disposal option. In fact, biological treatment of sulfides works quite well. However, the high pH of the sour water obtained from this type of olefin sour water stripper is likely to kill the microorganisms intended to digest the sulphur compounds. Some biological treatment operations are customized to moderate the concentration of the high pH sour water by diluting the sour water with other waste water streams. However, facilities producing large volumes of sour water may not have adequate volumes of non-sour waste water streams to sufficiently dilute the caustic stream. Outright pH neutralization of the sour water by adding an acid to the sour water is not at all desirable. The pH neutralization would volatilize the sulfides in the solution and release hazardous, obnoxious smelling hydrogen sulfide gas.
Industries have attempted to reduce the concentration of sulfides present in various materials. For example, U.S. Pat. No. 4,473,115 to Oakes describes a method for reducing concentrations of hydrogen sulfide in subterranean well fluids by injecting a mixture of "stabilized" chlorine dioxide into the well. The '115 patent describes stabilized chlorine dioxide as a solution prepared by adding separately to water:
(1) a source of active oxygen, including among various materials, hydrogen peroxide;
(2) a salt, such as an alkali metal carbonate; and
(3) chlorine dioxide gas which is added by bubbling it through the solution. The patent states that contacting the hydrogen sulfide in the drilling mud with the stabilized chlorine dioxide solution converts the hydrogen sulfide into insoluble colloidal sulfur.